It is known that oils used for edible purposes as well as those used industrially for mechanical applications are subject to an aging process which is influenced by high temperatures, among other things. Various chemical reactions take place, altering the quality of the particular oil.
Oil is frequently evaluated first on the basis of the visual impression, i.e., a decline in optical transmittance, or increasing discoloration over a period of time.
However, this variable is only a single parameter which is not generally sufficient for evaluating quality.
For example, a loss of quality in deep-frying fat is often discernible on the basis of other parameters even before there is any visible discoloration and may necessitate replacement of the fat.
The relevant quality of an oil may be ascertained, for example, by chemical tests, also in combination with visual tests.
For some time now, it has also been known that the aging condition of an oil may be evaluated on the basis of the measured dielectric constant.
One problem here is the fact that the dielectric constant also depends on the temperature. To solve this problem, for example, it is possible to provide for an oil sample to be heated or cooled to a fixedly predetermined temperature so that a dielectric measurement may be performed at this temperature. Such a measuring method is taken into account in U.S. Pat. No. 5,818,731, for example, as the related art.
The above publication also describes measuring methods by which the quality of an oil is to be determined by using several measured physical parameters such as a dielectric measured variable and the viscosity of the oil.
Since the color of a deep-frying fat is one of the most sensitive variables for determining its quality, U.S. Pat. No. 5,818,731 proposes combining a measurement of the dielectric constant with a measurement of optical transmittance in a certain wavelength range to perform a comprehensive evaluation of the quality of oil. To do so, a sample of an oil is placed in a graduated container, where it is exposed to light of a wavelength of 675 nanometers from a laser diode to measure the transmittance in this wavelength range. In addition, a measuring capacitor is used to measure the dielectric constant. This measurement is performed after the sample has been heated to a temperature between 155° C. and 185° C. After the dielectric constant has been measured, the measured dielectric constant is converted to the value at a standard temperature between 155° C. and 185° C. with the help of a temperature measurement and a stored regression curve on the basis of the known temperature dependence. This value should then allow a conclusion to be drawn regarding the quality of the fat in conjunction with the measured transmittance.
One disadvantage of the known measuring systems is that the measuring time amounts to several minutes up to approximately ten minutes and a certain quantity of the oil used must be taken as a sample and warmed to perform the measurement. The specimen container must then be cleaned thoroughly for a new measurement.
Accordingly, it is desirable to create a measuring system and a sensor system so that they will be simple to construct and will allow a rapid determination of the quality of the particular fluid while being easy and uncomplicated to operate.